When performing autonomous takeoff and/or landing, an important piece of information is the altitude of the aircraft. Some types of altitude sensors, such as sensors which use radar to measure altitude, have a “dead zone” near the ground where the sensors produce unreliable, inaccurate, and/or erroneous altitude measurements. To compensate for this, some autonomous takeoff and/or landing systems use two types of altitude sensors: one for altitude measurements at lower altitudes and another for altitude measurements at higher altitudes. This adds to the weight and cost of the aircraft, which is undesirable. New techniques for performing autonomous takeoff and/or landing that can accommodate an altitude-measurement dead zone and which are lighter and/or less expensive would be desirable.